Valence Electrons - MCAT | Premier MCAT Prep

Valence Electrons

7 min read Updated Mar 26, 2026

Not all electrons are created equal. The electrons buried deep inside an atom (core electrons) are tightly bound and chemically inert. The electrons in the outermost energy shell (valence electrons) are the ones that interact with other atoms - forming bonds, getting transferred, and determining chemical behavior.

What Makes an Electron a Valence Electron?

Valence electrons are the electrons in the outermost (highest n) energy shell. They are the least tightly held and the most available for bonding.

For main group elements (Groups 1-2 and 13-18), identifying valence electrons is straightforward:

The group number tells you the number of valence electrons
Group 1 (alkali metals): 1 valence electron (in the s subshell)
Group 2 (alkaline earth metals): 2 valence electrons (in the s subshell)
Group 13: 3 valence electrons (2 in s, 1 in p)
Group 14: 4 valence electrons (2 in s, 2 in p)
Group 15: 5 valence electrons
Group 16: 6 valence electrons
Group 17 (halogens): 7 valence electrons
Group 18 (noble gases): 8 valence electrons (except He with 2)

Transition Metals

For transition metals (d-block elements), valence electrons include electrons in both the highest s subshell AND the highest d subshell, even though they have different principal quantum numbers. For example:

Iron [Ar] 4s²3d⁶: valence electrons = 2 (from 4s) + 6 (from 3d) = 8 valence electrons
Vanadium [Ar] 4s²3d³: valence electrons = 2 + 3 = 5 valence electrons

Lanthanides and Actinides

For lanthanides and actinides (f-block elements), valence electrons include those in the highest s subshell AND the f subshell.

Core Electrons

All electrons that are NOT valence electrons are core electrons. They are in completely filled inner shells and do not participate in chemical reactions. Core electrons shield the valence electrons from the full charge of the nucleus - this concept of electron shielding is critical for understanding periodic trends (covered in Chapter 2).

The Octet Rule

Most atoms bond to achieve eight valence electrons (an octet) in their outermost shell, mimicking the electron configuration of the nearest noble gas. This is the octet rule, and it drives the vast majority of chemical bonding covered on the MCAT.

Elements in period 3 and below can exceed eight valence electrons by using their empty d orbitals. This is called an expanded octet and is covered in Chapter 3 (Bonding).

How many valence electrons does selenium (Group 16) have? What about its core electrons?

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6 valence electrons, 28 core electrons. Selenium (Z = 34) is in Group 16, so it has 6 valence electrons (4s²4p⁴). The remaining 34 - 6 = 28 electrons are core electrons. Note: selenium's 3d electrons are core electrons, not valence electrons, because the d subshell is fully filled and in a lower principal energy level.

Why do noble gases not form bonds under normal conditions?

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Noble gases already have a full octet (8 valence electrons, or 2 for helium). They have no driving force to gain, lose, or share electrons. Their electron configuration is maximally stable - every subshell in their outermost shell is filled. This is why the octet rule exists: other elements bond to achieve this same stable arrangement.